Chapter 4 Mobile Radio Propagation (Large-scale Path Loss)
-50 +90
-60 +80 Multi-path Propagation -70 +70 Field -80 +60 Strength, RSSI, dBuV/m dBm -90 +50 -100 +40
-110 +30
-120 +20 0 3 6 9 12 15 18 21 24 27 30 33 Distance from Cell Site, km
measured signal strength
signal strength predicted by Okumura- Hata propagation model
Wireless Communication
Objectives
To refresh understanding of basic concepts and tools To discuss the basic philosophy of propagation prediction applicable to cellular systems To identify and explore key propagation modes and their signal decay characteristics To discuss the multi-path propagation environment, its effects, and a method of avoiding deep fades To survey key available statistical propagation models and become familiar with their basic inputs, processes, and outputs To understand application of statistical confidence levels to system propagation prediction To review and gain familiarity with general measurement and propagation prediction tools available commercially
Chapter 4 –Mobile radio propagation( Large-scale path loss) 1 Dr. Sheng-Chou Lin
Page 1 Wireless Communication
Wave Propagation Basics: Frequency and Wavelength Wavelength is an important variable in RF propagation. /2 Wavelength determines the approximate required size of antenna elements.
Objects bigger than roughly a wavelength can reflect or block RF energy.
RF can penetrate into an enclosure if it has holes roughly a wavelength in size, or larger.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 2 Dr. Sheng-Chou Lin
Wireless Communication
Wave Propagation: Frequency and Wavelength
Radio signals travel through empty space at the speed of light ( Cell C) •C = 186,000 miles/second (300,000,000 meters/second) Frequency (F) is the number of waves per second (unit: Hertz) speed = C Wavelength (length of one wave) is Examples: calculated: •(distance traveled in one second) /(waves AMPS cell site f = 870 MHz. in one second) 0.345 m = 13.6 inches
PCS-1900 site f = 1960 MHz. C/F 0.153 m = 6.0 inches
Chapter 4 –Mobile radio propagation( Large-scale path loss) 3 Dr. Sheng-Chou Lin
Page 2 Wireless Communication
Statistical Propagation Models
Prediction of Signal Strength as a function of distance without regard to obstructions or features of a specific propagation path
-50 +90
-60 +80
-70 +70 Field -80 +60 Strength, RSSI, dBm dBuV/m -90 +50
-100 +40
-110 +30
-120 +20 0 3 6 9 12 15 18 21 24 27 30 33 Distance from Cell Site, km
measured signal signal strength predicted by Okumura- strength Hata propagation model
Chapter 4 –Mobile radio propagation( Large-scale path loss) 4 Dr. Sheng-Chou Lin
Wireless Communication
Radio Propagation
Mobile radio channel •fundamental limitation on the performance of wireless communications.s •severely obstructed by building, mountain and foliage. •speed of motion •a statistical fashion Radio wave propagation characteristics •reflection, diffraction and scattering •no direct line -of-sight path in urban areas •multipath fading Basic propagation types •Propagation model: predict the average received signal strength •Large-scale fading: Shadowing fading •Small-scale fading: Multipath fading
Chapter 4 –Mobile radio propagation( Large-scale path loss) 5 Dr. Sheng-Chou Lin
Page 3 Wireless Communication
Propagation Model
To focus on predicting the average received signal strength at a given distance from the transmitter •variability of the signal strength •is useful in estimating the radio coverage. Large-scale propagation •computed by averaging over 540, 1m 10m, for1GHz 2GHz. Small-scale fading •received signal strength fluctuate rapidly, as a mobile moves over very small distance. •Received signal is a sum of multi-path signals. •Rayleigh fading distribution •may vary by 30 40 dB •due to movement of propagation related elements in the vicinity of the receiver.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 6 Dr. Sheng-Chou Lin
Wireless Communication
Deterministic Techniques Basic Propagation Modes There are several very commonly- occurring modes of propagation, Free Space depending on the environment through which the RF propagates. Three are shown at right: •these are simplified, practically- calculable cases Reflection •real-world paths are often dominated by with partial cancellation one or a few such modes –these may be a good starting point for analyzing a real path –you can add appropriate corrections Knife-edge for specific additional factors you Diffraction identify •we’re going to look at the math of each one of these
Chapter 4 –Mobile radio propagation( Large-scale path loss) 7 Dr. Sheng-Chou Lin
Page 4 Wireless Communication
Free-Space Propagation
Effective area (Aperture) Aeff = A d Aeff ratio of power delivered to the Pt antenna terminals to 1 the incident power Gt 2 Gr density •: Antenna efficiency P t S = Gt : power density 4 d2 •A : Physical area Transmitter antenna P r = S A eff : Received power gain = Gt 2 4A eff Receiver antenna gain A eff = Gr G = = G 4 2 r 2 Propagation distance = P t P r = Gt G r d 4 d2 4 Wave length =
EIRP = PtGt (Effective isotropic radiated power)
Chapter 4 –Mobile radio propagation( Large-scale path loss) 8 Dr. Sheng-Chou Lin
Wireless Communication
Free-Space Propagation
A clear, unobstructed Line-of-sight path between them •Satellite communication, Microwave Line-of-sight (Point-to-point)
antenna 1 G1 frequency f or wavelength antenna 2 G2
P Pt r distance d
EIRP= PtGt = effective isotropic radiated power (compared to an isotropic radiator) : dBi ERP = EIRP-2.15dB = effective radiated power (compared to an half-wave dipole antenna) : dBd Path Gain P 2 8 2 r 2 c 3 10 gain = ------= G G ------ = G G ------ = G G ------ P 1 24d 1 24df 1 2 3 6 t 4d 1 10 f 1 10 for d in km, f in MHz
Path Loss = 1 / (Pr/Pt) when antenna gains are included
loss(dB) = 32.44 + 20 log d + 20 log f –G 1 dB –G2 dB Chapter 4 –Mobile radio propagation( Large-scale path loss) 9 Dr. Sheng-Chou Lin
Page 5 Wireless Communication
Free-Space Propagation
The simplest propagation mode r •Imagine a transmitting antenna at the center of an empty sphere. Each little square of surface intercepts its share of the radiated energy •Path Loss, db (between two isotropic antennas) = 36.58 +20*Log10(FMHZ)+20Log10(DistMILES ) Free Space •Path Loss, db (between two dipole antennas) “Spreading”Loss = 32.26 +20*Log10(FMHZ)+20Log10(DistMILES ) energy intercepted •Notice the rate of signal decay: by the red square is • 6 db per octave of distance change, which is 20 db per decade of distance change proportional to 1/r2 When does free-space propagation apply? 1st Fresnel Zone •there is only one signal path (no reflections) d •the path is unobstructed (first Fresnel zone is not P penetrated by obstacles) A D First Fresnel Zone = B {Points P where AP + PB - AB < } Fresnel Zone radius d = 1/2 (D)^(1/2)
Chapter 4 –Mobile radio propagation( Large-scale path loss) 10 Dr. Sheng-Chou Lin
Wireless Communication Near and Far fields
D
/2 radiated 2 0 reactive 2D / radiated far field near field
These distances are rough approximations! Reactive near field has substantial reactive components which die out Radiated near field angular dependence is a function of distance from the antenna (i.e., things are still changing rapidly) Radiated far field angular dependence is independent of distance Moral: Stay in the far field!
Chapter 4 –Mobile radio propagation( Large-scale path loss) 11 Dr. Sheng-Chou Lin
Page 6 Wireless Communication
An Example
An antenna with maximum dimension (D) of 1m, operating frequency (f) = 900 MHz. •= c/f = 3108/900 106 = 0.33 2 2 •Far-field distance = df = 2D / = 2 (1) /0.33 = 6m
TX power, Pt = 50W, fc = 900MHz, Gt = 1 = Gr 3 •Pt (dBm) = 10log(50 10 mW) = 47 dBm = 10lon(50) = 17 dBW
•Gt = 1 = Gr = 0dB
•Loss (100m)= 32.44 +20log(dkm)+20log(fMHz) = 32.44 + 20log (0.1)+20log(900) =71.525 dB
–Pr (100m)= 47 +0 –71.525 +0 = -24.5 dBm
•Loss (10km)= 32.44 +20log(dkm)+20log(fMHz) = 32.44 + 20log (10)+20log(900) =71.525 dB + 40 = 111.525 dB
–Pr (100m)= 47 +0 –111.525 +0 = -64.5 dBm
Chapter 4 –Mobile radio propagation( Large-scale path loss) 12 Dr. Sheng-Chou Lin
Wireless Communication
A Tedious Tale of One Radio Link
Transmitter Let’s track the power flow from 20 Watts TX output transmitter to receiver in the x 0.50 line efficiency radio link we saw back in lesson Trans. = 10 watts to antenna x 20 antenna gain 2. We’re going to use real values Line = 200 watts ERP that commonly occur in typical x 0.000,000,000,000,000,1585links.path attenuation Antenna = 0.000,000,000,000,031,7 watts if intercepted by dipole antenna
x 20 antenna gain = 0.000,000,000,000,634 watts into line
Antenna x 0.50 line efficiency = 0.000,000,000,000,317 watts to receiver Trans. Line Did you enjoy that arithmetic? Let’s go back and do it again, a better and less painful way.
Receiver
Chapter 4 –Mobile radio propagation( Large-scale path loss) 13 Dr. Sheng-Chou Lin
Page 7 Wireless Communication
Decibels - A Helpful Convention
x 1000 Decibels normally refer to power ratios -- in other words, the numbers we represent in dB usually .001 w 1 watt are a ratio of two powers. Examples: 0 dBm 30 dBm •A certain amplifier amplifies its input by a factor of +30 dB 1,000. (Pout/Pin = 1,000,000). That amplifier has 30 dB gain. x 0.10 10 w •A certain transmission line has an efficiency of only 10 100 w percent. (P /P = 0.1) The transmission line has a +50 dBm +40 dBm out in -10 dB loss of -10 dB. Often decibels are used to express an absolute • dB are comfortable-size numbers • rather than multiply and divide RF number of watts, milliwatts, kilowatts, etc. When power ratios, in dB we can just add used this way, we always append a letter (W, m, or & subtract • Given a number, convert to dB: K) after “db”to show the unit we’re using. For
db = 10 x Log10 (N) example, • Given dB, convert to a number: •20 dBK = 50 dBW = 80 dBm= 100,000 watts N = 10^(db/10) •0 dBm = 1 milliwatt
Chapter 4 –Mobile radio propagation( Large-scale path loss) 14 Dr. Sheng-Chou Lin
Wireless Communication A Much Less Tedious Tale of that same Radio Link
Transmitter +43 dBm TX output Let’s track the power flow again, using decibels. Trans. -3 dB line efficiency Line = +40 dBm to antenna +13 dB antenna gain = +53 dBm ERP Antenna -158 dB path attenuation = -105 dBm if intercepted by dipole antenna (+4.32dB for EIRP)
+13 dB antenna gain = -92 dBm into line Antenna -3 dB line efficiency = -95 dBm to receiver Trans. Line Wasn’t that better?! Let’s look at how dB work.
Receiver
Chapter 4 –Mobile radio propagation( Large-scale path loss) 15 Dr. Sheng-Chou Lin
Page 8 Wireless Communication
Introduction The Function of an Antenna
Antenna 1 Antenna 2
Transmission Electromagnetic Transmission Line Field Line RF RF Power Power current current Available
An antenna is a passive device (an arrangement of electrical conductors) which converts RF power into electromagnetic fields, or intercepts electromagnetic fields and converts them into RF power. RF power causes current to flow in the antenna. The current causes an electromagnetic field to radiate through space. The electromagnetic field induces small currents in any other conductors it passes. These currents are small, exact replicas of the original current in the original antenna.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 16 Dr. Sheng-Chou Lin
Wireless Communication
Antenna Polarization Antenna 1 Vertically Antenna 2 Polarized Horizontally Polarized Transmission Electromagnetic Transmission Line Field Line RF RF Power Power current Available almost no current
Antenna 1
The electromagnetic field is oriented by the direction of current flow in the radiating antenna. To intercept significant energy, a receiving antenna should be oriented parallel to the transmitting antenna. A receiving antenna oriented at right angles to the transmitting antenna will have very little current induced in it. This is referred to as cross- polarization. Typical cross-polarization loss is 20 dB. Vertical polarization is the norm in mobile telephony.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 17 Dr. Sheng-Chou Lin
Page 9 Wireless Communication
Reference Antennas and Effective Radiated Power
Effective Radiated Power is always expressed in relation to the radiation produced by a reference antenna. Isotropic The flashlight example used a plain light bulb as Antenna a reference - producing the same light in all directions. The radio equivalent of a plain light bulb is called an isotropic radiator. It radiates the same in all directions. Unfortunately, it virtually impossible to build such an antenna. •Radiation compared to an isotropic radiator is called Dipole Antenna EIRP, Effective Isotropic Radiated Power. Null The simplest, most common, physically constructible reference antenna is a dipole. Main •Radiation compared to a dipole is called ERP, Effective Lobe Radiated Power. Null
Chapter 4 –Mobile radio propagation( Large-scale path loss) 18 Dr. Sheng-Chou Lin
Wireless Communication
Reference Antennas, ERP and EIRP Dipole Null
ERP is by comparison to a Dipole Main Lobe •This is the tradition in cellular, land mobile, HF Null communications, and FM/TV broadcasting Isotropic EIRP is by comparison to an Isotropic Radiator •This is the tradition in PCS at 1900 MHz., microwave, satellite communications, and radar ERP values can be converted to EIRP and vice versa. •For a given amount of power input, a dipole produces 2.16 db more radiation than an isotropic radiator, due to the dipole slight directionality. A third antenna compared against both dipole and isotropic will have a bigger EIRP (vs. isotropic) than ERP (vs dipole). The difference is 2.16 db, a power ratio of 1.64. Therefore,
ERP = EIRP - 2.16 dB and ERP = EIRP / 1.64 EIRP = ERP + 2.16 dB and EIRP = ERP x 1.64
Chapter 4 –Mobile radio propagation( Large-scale path loss) 19 Dr. Sheng-Chou Lin
Page 10 Wireless Communication
Radiation Patterns Key Features and Terminology
Radiation patterns of antennas are usually Typical Example plotted in polar form Horizontal Plane Pattern The Horizontal Plane Pattern shows the Notice -3 dB points radiation as a function of azimuth 0 (N) (i.e.,direction N-E-S-W) 0 10 db The Vertical Plane Pattern shows the -10 points radiation as a function of elevation -20 Main (i.e., up, down, horizontal) -30 db Lobe Antennas are often compared by 270 90 noting specific features on their (W) nulls or a Minor (E) patterns: minima Lobe •-3 db (“HPBW”), -6 db, -10 db points Front-to-back Ratio •front-to-back ratio •angles of nulls, minor lobes, etc. 180 (S)
Chapter 4 –Mobile radio propagation( Large-scale path loss) 20 Dr. Sheng-Chou Lin
Wireless Communication
Two Basic Methods of Obtaining Gain
Quasi-Optical Techniques (reflection, focusing) •Reflectors can be used to concentrate radiation –technique works best at microwave frequencies, where reflectors are small •examples: –corner reflector used at cellular or higher frequencies –parabolic reflector used at microwave frequencies –grid or single pipe reflector for cellular Array Techniques (discrete elements) •power is fed or coupled to multiple antenna elements; each element radiates In Phase •elements?radiations in phase in some directions •in other directions, different distances to distant observer introduce different phase delay for each Out of element, and create pattern lobes and nulls Phase
Chapter 4 –Mobile radio propagation( Large-scale path loss) 21 Dr. Sheng-Chou Lin
Page 11 Wireless Communication
Real-World Path Loss
Free space is, in general, NOT the real world. We must deal with:
•reflections over flat or curved Earth •reflections from smooth •scattering from rough surfaces •diffraction around/over obstacles •absorption by vegetation and other lossy media, including buildings and walls •multipath fading •approximately fourth power propagation loss
Chapter 4 –Mobile radio propagation( Large-scale path loss) 22 Dr. Sheng-Chou Lin
Wireless Communication
Reflection
A propagating wave impinges upon an object with very large dimensions ( >> ) Reflections occur from surface of the earth and from building and wells Flat surface
Path Loss (dB) = 40Log(d) - [10Log(Gt )+10Log (Gr ) +20Log (hb ) +20Log (hm )]
2 2 hb hm P P G G r = t t r 4 TX ERPDBM d
= r1 + r2 - r = phase difference in two paths h - h r b m h b r1
r2 d
Chapter 4 –Mobile radio propagation( Large-scale path loss) 23 Dr. Sheng-Chou Lin
Page 12 Wireless Communication Reflection with Partial Cancellation
Assumptions: •the cell is a mile away or more •the cell is not over a few hundred feet Direct ray higher than the car •there are no other obstructions Reflected If these assumptions are true, then: Ray •The point of reflection will be very close to the car -- at most, a few hundred feet away. •the difference in path lengths is influenced Point of most strongly by the car antenna height reflection above ground or by slight ground height variations The reflected ray tends to cancel the This reflection is at “grazing incidence”. The reflection is virtually 100% efficient, direct ray, dramatically reducing the and the phase of the reflected signal flips received signal level 180 degrees.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 24 Dr. Sheng-Chou Lin
Wireless Communication
Reflection with Partial Cancellation Analysis: •physics of the reflection cancellation TX ERPDBM predicts signal decay approx. 40 db per decade of distance –twice as rapid as in free-space! HTFT HT •observed values in real systems range FT from 30 to 40 db/decade Received Signal Level, dBm =
TX ERPDBM - 172 DMILES - 34 x Log10 (DMILES ) + 20 x Log10 (Base Ant. HtFEET) + 10 x Log10 (Mobile Ant. HtFEET) Comparison of Free-Space and Reflection Propagation Modes Assumptions: Flat earth, TX ERP = 50 dBm, @ 870 MHz. Base Ht = 200 ft, Mobile Ht = 5 ft.
DistanceMILES 1 2 4 6 8 10 15 20
FS usingFree-SpaceDBM -45.3 -51.4 -45.3 -57.4 -63.4 -65.4 -68.9 -71.4
FS using ReflectionDBM -69.0 -79.2 -89.5 -95.4 -99.7 -103.0 -109.0 -113.2
Chapter 4 –Mobile radio propagation( Large-scale path loss) 25 Dr. Sheng-Chou Lin
Page 13 Wireless Communication
Observation on Signal Decay Rates
Signal Level vs. Distance We’ve seen how the signal decays 0 with distance in two simplified modes of propagation: -10 Free-Space •20 dB per decade of distance -20 •6 db per octave of distance Reflection Cancellation -30 •40 dB per decade of distance •12 db per octave of distance -40 1 2 3.16 5 6 7 8 10 Real-life cellular propagation decay Distance, Miles rates are typically somewhere One Decade between 30 and 40 dB per decade One Octave of distance (10x) of distance (2x) of distance
Chapter 4 –Mobile radio propagation( Large-scale path loss) 26 Dr. Sheng-Chou Lin
Wireless Communication
Diffraction
Diffraction allows radio signals to propagate around the curved surface of the earth, beyond the horizon, and to propagate behind obstructions. •The diffraction field still exists and often has sufficient strength to produce a useful signal, as a receiver moves deeper into the obstructed (shadowed) region. •Caused by the propagation of secondary wavelets into a shadowed region. •Sum of the electric field components of all the secondary wavelets in the space around the obstacle. Excess path length ( ): the difference between the direct path and the diffracted path. •A function of height and position of the obstruction, as well as the transmitter and receiver location. Fresnel zones: successive receiver where = n/2 •provide constructive and destructive interference to the total received signal. •Obstruction does not block the volume within the first Fresnel zone.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 27 Dr. Sheng-Chou Lin
Page 14 Wireless Communication
Diffraction parameter
2 Excess path length (difference between h ( d1 + d2 ) = direct path and diffracted path) 2 d1 d2 2 2 2 h ( d1 + d2 ) •The corresponding phase difference = = 2 d1 d2 d + d •= + h ( 1 2 ) d1 d2 2 d1 d2 •Fresnel-Kirchoff diffraction Parameter = ( d + d ) 1 2
= 2 2 h Phase difference bet. LOS and diff. Path is a function of height and position of the obstruction, as well as TX and RX d1 d2
Chapter 4 –Mobile radio propagation( Large-scale path loss) 28 Dr. Sheng-Chou Lin
Wireless Communication Fresnel Zones R
) 0 d1 d2 B d (
s 4 s o l 8 n o i t c
a 12 r
Rx f f
Tx i obstacle D 16 nd d Fresnel zones are the family of ellipsoids 1 2 Rn = certain values of phase of the rays which (d1+d2) h 0 which are the loci of circles that indicate as = n h = 0 is the direct ray pass through them or = n /2 Generally want antenna heights high enough so all obstacles are below first Fresnel zone (n = 1) If tip of obstacle is at center of Fresnel zone (LOS ray), then loss is 6 dB greater than free-space path loss
d1d2 R = R is 1st Fresnel Zone radius, d1,d2 in km, and f in GHz (d1+d2) f
Chapter 4 –Mobile radio propagation( Large-scale path loss) 29 Dr. Sheng-Chou Lin
Page 15 Wireless Communication Knife-Edge Diffraction
Radio signals to propagate between Transmitter H and Receiver is obstructed by a surface that has sharp irregularities (edges) such as hill or mountain.
R1 R2 Sometimes a single well-defined obstruction blocks the path. This case is fairly easy to analyze 2 1 1 and can be used as a manual tool to estimate the = H effects of individual obstructions. R1 R2 First calculate the parameter from the geometry of the path Ed /Eo = F(), Gd,dB= 20logF() Next consult the table to obtain the obstruction 0 loss in db -5 Add this loss to the otherwise-determined path -10 atten loss to obtain the total path loss. dB -15 -20 Other losses such as reflection cancellation still -25 apply, but computed independently for the path sections before and after the obstruction. -5 -4 -3 -2 -1 0 1 2 3 Chapter 4 –Mobile radio propagation( Large-scale path loss) 30 Dr. Sheng-Chou Lin
Wireless Communication An Example of Diffraction
= 1/3 m, d1 = 1km, d2 =1km,and(a)h=25m(b)h =0(c)h= -25m.
2 d1 d2 (a) h = 25m: = 2.74, Loss = 22 dB from = Figure 4.14. Approximation = 21.7 dB, = ( d + d ) 1 2 0.625m, = 1/3, n = 3.75 the tip of the 2 obstruction completely blocks the first h ( d1 + d2 ) = three Fresnel zones. 2 d1 d2 (b) h = 0m: = 0, Loss = 6 dB from Figure 4.14. Approximation = 6 dB, = 0m the = n or = n /2 for Fresnel tip of the obstruction lies in the middle of Zones the first Fresnel zone.
(c) h = -25m: = -2.74, Loss = 1dB from h Figure 4.14. Approximation = 0dB, = 0.625m, = 1/3, n = 3.75 the tip of the obstruction completely blocks the first three Fresnel zones. However, the diffraction losses are negligible, since the d d 1 2 obstruction is below the LOS.
Chapter 4 –Mobile radio propagation( Large-scale path loss) 31 Dr. Sheng-Chou Lin
Page 16 Wireless Communication
Scattering
Why consider scattering •Actual received signal what predicted by reflection and diffraction •Rough surface reflected energy is spread out (diffused) •Flat surface with dimensions . •number of obstacles per unit volume is large. •Rough surfaces, small objects irregularities scattering •ex. Foliage, trees, , street signs, lamp post.
Rayleigh criterion
•Rough surface: h > hc, hc = / 8sinI
•reflection coefficient = flat coefficient s
–rough = s
–s : scattering loss
Chapter 4 –Mobile radio propagation( Large-scale path loss) 32 Dr. Sheng-Chou Lin
Wireless Communication
Propagation Model
General types •Outdoor •Indoor : conditions are much more variable. Most of these models are based on a systematic interpretation of measurement data obtained in the service area. Parameters used in propagation model •Frequency •Antenna heights •Environments : Large city, medium city, suburban, Rural (Open) Area. Common models •Hata Model : 20km > Range >1km •Walfisch and Bertoni Model :Range < 5km •Indoor propagation models : include scattering, reflection, diffraction –conditions are much more variable
Chapter 4 –Mobile radio propagation( Large-scale path loss) 33 Dr. Sheng-Chou Lin
Page 17 Wireless Communication
Statistical Propagation Models
Based on statistical analysis of large amounts of measurement data Predict signal strength as a function of distance and various parameters Useful for early network dimensioning, number of cells, etc. “Blind”to specific physics of any particular path -- based on statistics only Easy to implement as a spreadsheet on PC or even on hand- held programmable calculator Very low confidence level if applied as spot prediction method, but very good confidence level for system-wide generalizations
Chapter 4 –Mobile radio propagation( Large-scale path loss) 34 Dr. Sheng-Chou Lin
Wireless Communication
Statistical Propagation Models
Prediction of Signal Strength as a function of distance without regard to obstructions or features of a specific propagation path
-50 +90
-60 +80
-70 +70 Field -80 +60 Strength, RSSI, dBm dBuV/m -90 +50
-100 +40
-110 +30
-120 +20 0 3 6 9 12 15 18 21 24 27 30 33 Distance from Cell Site, km measured signal strength signal predicted by Okumura- strength Hata propagation model Chapter 4 –Mobile radio propagation( Large-scale path loss) 35 Dr. Sheng-Chou Lin
Page 18 Wireless Communication Statistical Propagation Models: Commonly-required Inputs
Frequency Distance from transmitter to receiver Effective Base Station Height Average Terrain Elevation Arbitrary loss allowances based on rules-of-thumb for type of area (Urban, Suburban, Rural, etc.) Arbitrary loss allowance for penetration of buildings/vehicles Assumptions of statistical distribution of variation of field strength values
Chapter 4 –Mobile radio propagation( Large-scale path loss) 36 Dr. Sheng-Chou Lin
Wireless Communication
Okumura Model
L50 (dB) = LF +Amu (f,d) –G(ht) –G(hr) –GAREA
• Widely used model for signal prediction in urban areas • is based on measured data and does not provide any analytical explanation
Where: L50 = The 50% (median) value of propagation path loss LF = The free space propagation loss Amu (f,d) = median attenuation relative to free space (see Fig. 3.23) G(ht) = Base station antenna height gain factor (30m ~1000m) G(hr) = mobile antenna height gain factor GAREA = Gain due to the type of environment (see Fig. 3. 24)
f : 150MHz ~ 1920MHz (up to 3000MHz), d: 1km ~ 100km
G(ht) = 20log ( ht /200 ), G(hr) = 10log ( hr /3 ), hr 3m G(hr) = 20log ( hr /3 ), 10m hr 3m
Chapter 4 –Mobile radio propagation( Large-scale path loss) 37 Dr. Sheng-Chou Lin
Page 19 Wireless Communication An Example using Okumura Model
D= 50 km, ht = 100m, hr = 10m, in an urban environment. EIRP = 1kW, f = 900 MHz, unit gain receiving antenna.
• LF = 125.5dB
• Amu(900MHz, 50 km)) = 43 dB
• GAREA = 9dB
• G(ht) = -6dB
• G(hr) = 10.46 dB
• L50 = 155.04 dB
• Pr(d) = 60-155.04 + 0 = -95.04 dBm
Chapter 4 –Mobile radio propagation( Large-scale path loss) 38 Dr. Sheng-Chou Lin
Wireless Communication
Hata Model
L50 (Urban) (dB) = 69.55 + 26.16 log (F) –13.82 log(Hb) + (44. 9 –6.55 log(Hb) )*log (D) –a
Where: A = Path loss F = Frequency in mHz (150M-1500 MHz) D = Distance between base station and terminal in km (1km ~20km) H = Effective height of base station antenna in m (30m ~200m) a = Environment correction factor for mobile antenna height (1m~10m)
a = (1.1 log (F) - 0.7) Hm - (1.56 log (F)- 0.8 ) dB = Small~medium sized city (urban)
8.29 (log ( 1.54 Hm)) 2 - 1.1 dB for F 300 MHz = Large city (Dense 2 3.2 log (F) (log (11.75 Hm)) - 4.97 dB for F 300 MHz Urban)
2 • L50 (Urban) - 2(log(F/28)) - 5.4 = Suburban 2 • L50 (Urban) - 4.78(log(F)) - 18.33 (log(F)) - 40.98 = Rural (open)
• L90 = L50 + 10.32 dB : 90% QOS, L50 is the median value of propagation loss
Chapter 4 –Mobile radio propagation( Large-scale path loss) 39 Dr. Sheng-Chou Lin
Page 20 Wireless Communication
COST-231 Hata Model
A (dB) = 46.3 + 33.9log (F) –13.82 log(Hb) + (44. 9 –6.55 log(Hb) )*log (D) –a + c
Where: A = Path loss F = Frequency in MHz (1500M-2000 MHz) D = Distance between base station and terminal in km (1km ~20km) H = Effective height of base station antenna in m (30m ~200m) a = Environment correction factor for mobile antenna height c = Environment correction factor
C = 0 dB = Small~medium sized city (urban), Suburban
3 dB = Dense Urban (metropolitan center)
A is defined in the Hata Model
Chapter 4 –Mobile radio propagation( Large-scale path loss) 40 Dr. Sheng-Chou Lin
Wireless Communication
Statistical Propagation Models Okumura-Hata Model
A (dB) = 69.55 + 26.16 log (F) –13.82 log(H) + (44. 9 –6.55 log(H) )*log (D) + C
Where: A = Path loss F = Frequency in MHz (800-900 MHz) D = Distance between base station and terminal in km H = Effective height of base station antenna in m C = Environment correction factor
C = 0 dB = Dense Urban - 5 dB = Urban - 10 dB = Suburban - 17 dB = Rural
Chapter 4 –Mobile radio propagation( Large-scale path loss) 41 Dr. Sheng-Chou Lin
Page 21 Wireless Communication
Statistical Propagation Models COST-231 HATA Model
A (dB) = 46.3 + 33.9*logF –13.82*logH + (44.9 –6.55*logH)*log D + C
Where: A = Path loss F = Frequency in mHz (between 1700 and 2000 mHz) D = Distance between base station and terminal in km H = Effective height of base station antenna in m C = Environment correction factor
C = - 2 dB = for dense urban environment: high buildings, medium and wide streets
- 5 dB = for medium urban environment: modern cities with small parks
- 8 dB = for dense suburban environment, high residential buildings. wide streets
- 10 dB = for medium suburban environment. industrial area and small homes
- 26 dB = for rural with dense forests and quasi no hills
Chapter 4 –Mobile radio propagation( Large-scale path loss) 42 Dr. Sheng-Chou Lin
Wireless Communication
Statistical Propagation Models Walfisch-Ikegami Model
Useful only in dense urban environments, but often superior to other methods in this environment Based on “urban canyon”assumption • a “carpet”of buildings divided into blocks by street canyons
-20 dbm • Uses diffraction and reflection mechanics and Signal Level -30 dbm statistics for prediction Legend -40 dbm -50 dbm -60 dbm • Input variables relate mainly to the geometry of the -70 dbm buildings and streets -80 dbm -90 dbm -100 dbm Useful for two distinct situations: -110 dbm Area View -120 dbm • macro-cell - antennas above building rooftops • micro-cell - antennas lower than most buildings Available in both 2-dimensional and 3-dimensional versions
Macrocell Microcell
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Statistical Techniques Practical Application of Distribution Statistics Percentage of Locations where Observed Technique: RSSI exceeds Predicted RSSI •use a model to predict RSSI 10% of locations exceed this RSSI •compare measurements with model 50% – obtain median signal strength RSSI, 90% – obtain standard deviation dBm – now apply correction factor to obtain field strength required for desired probability of service Applications: Given Distance •a desired signal level Normal Occurrences •the standard deviation of signal strength Distribution measurements •a desired percentage of locations which Median RSSI must receive that signal level Signal •We can compute a “cushion”in dB Strength , which will give us that % coverage dB
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Propagation Loss
Comparison among models •Free space •Hata Model (Okumura + COST 231) • Walfisch : considered by ITU-R in IMT-2000 standard. Cellular system, f = 850MHz PCS system, f =1900 MHz
Propagation Distance Propagation Distance 80.00 80.00 Dense Urban (Hata) Dense Urban (Hata) Urban (Hata) Urban (Hata) Suburban Suburban Rural 100.00 100.00 Rural Free Space Free Space Dense Urban (Walfish) Dense Urban (Walfish) Urban (Walfish) Urban (Walfish) 120.00 120.00 ) ) B B d d ( (
s s
s 140.00
s 140.00 o o L L
160.00 160.00
180.00 180.00
200.00 200.00 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Distance (km) Distance (km)
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Page 23 Wireless Communication Statistical Techniques Example of Application of Distribution Statistics Suppose you want to design a Standard Gaussian ( m = 0, =1) cell site to deliver at least -95 Cumulative Normal Distribution dBm to at least 90% of the 100% locations in an area 90% Measurements you’ve made 80% have a 10 dB. standard deviation 70% above and below the average 60% signal strength 50% On the chart: 40% •to serve 90% of possible 30% locations, we must deliver an
20% average signal strength 1.29 standard deviations stronger than 10% -95 dBm, = 10 0% •-95+(1.29x10)= - 82 dbm -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 Standard Deviations from •Design for an average signal Median (Average) Signal Strength strength of - 82 dbm!
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Statistical Techniques Normal Distribution Graph & Table for Convenient Reference
Cumulative Normal Distribution Standard Cumulative Deviation Probability 100% -3.09 0.1% 90% -2.32 1% 80% -1.65 5% 70% -1.28 10% 60% -0.84 20% -0.52 30% 50% 2.35 99% 40% 0 50% 30% 0.52 70% 20% 0.84 80% 1.28 90% 10% 1.65 95% 0% 2.35 99% -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.09 99.9% Standard Deviations from Mean Signal Strength
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Log-normal Shadowing Fading
Long-term variation are due to propagation through obstructions, ets. And if the number of obstructions is large
The loss in dB is respected as a Gaussian distribution with a mR(dB) and variance (dB)
•mR is the median loss of the path We choose a specific coverage criteria such as 95%, 90%, 85% . To find 90% Loss •Pr [ Loss m + Loss ()] = 90% R Normal Occurrences Distribution In all these cases, itself is a function of the environment •Large, medium city, suburban : 8dB Median RSSI Signal •Rural area 4dB Strength , –for = 9 dB.and 90% coverage, Loss ()= dB 10.32 dB
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Shadowing Fading statistics
Long-term variation is modeled by a log-normal distribution
- (j) d - d Ep = Eo e Ep = Eo e
•at the receiver, the input signal will be given by Eo Ep
- i ri - iri d Er = Ei e Er = Ei e
101og(Y) = 10log(e)X
•if number of obstructions is large, - I ri is Gaussianly distributed for any and r Normal I i Distribution •y = ex: , y is lognormally distributed if x is Gaussianly distributed. , dB Chapter 4 –Mobile radio propagation( Large-scale path loss) 49 Dr. Sheng-Chou Lin
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Percentage of Coverage
The percentage of area with a received signal , I.e. Pr [ Pr (R) ] •: desired received signal threshold •radial distance from the transmitter –received signal at D = R exceeds the threshold –see Fig. 3.18 for different n and Ex: shadowing deviation = 8dB, 75% boundary coverage (QOS) •Loss exponent factor n = 4 area coverage 94% •Loss exponent factor n = 2 area coverage 91%
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Statistical Propagation Models Typical Results
Example of Model Results: Typical Cell Range Predictions for Various Environments
Tower Height EIRP Range F = 1900 mHz (meters) (watts) (km) Dense Urban 30 200 1.05 Urban 30 200 2.35 Suburban 30 200 4.03 Rural 50 200 10.3
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Building Penetration Losses
Usual technique for path loss into a building: get median signal level in streets by some “normal”method add building penetration losses Loss 1/h in general Small scale variation is Rayleigh Large scale variation is log-normal Loss 1/f Each additional floor is about 2 dB difference in loss For primarily scattering paths, standard deviation is about 4 dB For paths with at least partial LOS, standard deviation is about 6 to 9 dB Windows of many new buildings have a thin layer of metal sputtered on the window glass; this increases attenuation
D. Molkdar, “Review on radio propagation into and within buildings,”IEE Proc-H, Vol. 138, No. 1, Feb 1991, pp 61- 73.
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Propagation Inside Buildings
Indoor environment differs from mobile environment in • interference environment (usually higher, due to equipment) • fading rate (usually slower, due to reduced speeds) Limitations due to bandwidth • narrowband (e.g. TDMA) systems coverage limited by multipath and shadow fading • wideband systems experience ISI due to delay spread (less frequency diversity gain) Power as a function of distance varies over a range: • P 1/d2 in a near-free-space environment (hallway) • P 1/d6 in a high-clutter environment (room full of cubes) Loss: floors with structural metal > brick wall > plaster wall Office fading is usually more continuous /smaller dynamic range than mobile fading Stairwells and elevator shafts can act as waveguides and aid floor-to-floor propagation Presence of an LOS path reduces RMS delay spread
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Page 27 Wireless Communication Propagation Inside Buildings - Prediction
Tx
actual
direct diffraction Rx number of oors dif fracted path direct path Combination of ray-tracing and diffraction can be very accurate at predicting inside propagation Direct rays (through floors): each floor increases loss Diffraction (windows and outside): large loss initially, but more floors do not add much loss 900 MHz band losses 10 dB/floor for reinforced concrete 13 dB/floor for precast slab floor 26 dB isolation for corrugated steel (diffraction path dominates)
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Acceptable Cellular Voice Quality
•AMPS VOICE QUALITY IS ACCEPTABLE IF OVER 90% OF THE COVERAGE: 1) VOICE S/N RATIO > 38 dB IN FADING ENVIRONMENT 2) RF CARRIER-TO-INTERFERENCE RATIO (CIR) > 18 dB •GIVEN 1) AND 2), 75% OF USERS GRADE THE SYSTEM AS “GOOD”OR “EXCELLENT” •MATHEMATICALLY:
S C = 10 log 3 C + 15 dB, where > 13 dB, and > 0.6 N 10 4 I I S = BASEBAND SIGNAL-TO-NOISE RATIO N C = RF CARRIER-TO-INTERFERENCE RATIO I •IN DIGITAL SYSTEMS WITH VOICE COMPRESSION, VOICE QUALITY IS USUALLY QUANTIFIED PSYCHOACOUSTICALLY VIA Mean Opinion Score (MOS) RATINGS ON ASCALE OF 1 TO 5.
•AN MOS SCORE OF 3 IS CONSIDERED MINIMALLY ACCEPTABLE Bernardin, C.P. et al ,"Voice Quality Prediction in AMPS Cellular Systems using SAT," Wireless 94 Symposium, Calgary, July 12, 1994, pp 238-241.
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Lesson 4 Complete
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